Contactless interrogation of sensors for smart structures

ABSTRACT

Apparatus for contactless interrogation of an sensor integrally disposed with a structure including a pair of coils for coupling signals across a gap between a sensing circuit having a sense coil and an interrogation circuit having an exciter coil connectable to a variable frequency supply, the sensing circuit comprising a circuit for changing current through the sense coil in relation to the sensor output; the interrogation circuit comprising a circuit for detecting current in the exciter coil induced by the sense coil current; and control means for determining the sensor output based on the exciter coil current compensated for the gap.

BACKGROUND OF THE INVENTION

The invention relates generally to apparatus and methods forinterrogating sensors, and more particularly the invention relates toapparatus and methods for interrogation of sensors embedded in ormounted on structures.

Significant advances have been made in developing new high performancematerials such as, for example, graphite/epoxy composites. Thesematerials and many other composite types hold great promise forreplacing conventional materials such as steel and aluminum instructures subjected to various environmental conditions such as hightemperature, pressure, stress and strain. For example, carbon compositematerials are expected to be used extensively in next generationaircraft for structures such as the wings and other air foil surfaces,engine drive shafts and so on just to name a few examples.

Although these materials have been shown to offer substantial benefitsover conventional materials, industry acceptance of these materials,especially in the aerospace industry, has been limited due to the lackof statistical databases on their failure modes because of theirrelatively recent introduction. This situation presents a catch-22because until such materials have been subjected to extensive use, suchdatabases will remain relatively unavailable. Also, the lack ofstatistical analysis results in structures using these materials beingoverdesigned to the point that the benefits of using the improvedmaterials are reduced or eliminated altogether.

Because of the need to characterize the real-world performance of thesematerials, extensive activity has been undertaken to develop "smart"structures in which the structures include embedded or integratedsensors that monitor one or more structural parameters such as stress,strain etc. Such smart structures are described, for example, in U.S.Pat. Nos. 4,983,034 and 4,930,852 issued to Spillman and Wheeler et al.respectively, and commonly owned by the assignee of the presentinvention, the entire disclosures of which are fully incorporated hereinby reference. The basic concept is that the integrated sensors can beused as health monitors for the structure to characterize the structuralperformance of the materials. Having such real time or near real timeinformation can allow structures to be designed up to the materiallimits. In fact, the structures can further include active elements thatprovide adaptive compensation for structure performance. Such a smartstructure concept is disclosed in U.S. Pat. No. 4,922,096 issued toBrennan and U.S. patent application Ser. No. 07/981,966 filed on Nov.25, 1992 for "Smart Structure With Non-Contact Power and DataInterface", which are commonly owned by the assignee of the presentinvention, the entire disclosures of which are fully incorporated hereinby reference.

Optical sensors such as those described in the referenced patents are adesirable solution to providing smart structures. Such sensorsconveniently use optic fibers as part of the active sensor element, andof course, optic fibers are small and flexible thus making them idealfor embeddment in composite structures without adversely affectingstructural integrity. For example, structural strain can be monitored bytransmitting light through one or more embedded optic fibers that bendunder stress, strain etc. thus affecting the transmissioncharacteristics of the light through the fiber.

The successful use of such smart structures, optics based or otherwise,requires a reliable and accurate way to interrogate the sensors. Opticalsensors present a particularly difficult ingress/egress data and powerproblem because light energy must be delivered to the sensor and theoutput light pattern or signals corresponding thereto must be coupledback out to the outside world for processing and analysis. Usinghardwired or other mechanical, electrical or optical connections betweenthe sensors and external hardware is difficult from a manufacturingstand point due to the need for precise machining and very tighttolerances to allow efficient coupling. Although it is generally knownto use RF coupling for contactless interrogation of some sensors, suchapproaches typically depend on frequency domain analysis (such as bydetecting a resonant frequency shift based on a sensed parameter.) Theseapproaches are not practical, therefore, for resistive sensors, nor forextracting data from optical sensors because these sensors produceamplitude dependent outputs.

Accordingly, the need exists for non-contact apparatus and methods forinterrogating smart structure sensors, particularly embedded sensors.Such needed apparatus and methods should also be convenient to use forresistive sensors and optical sensors.

SUMMARY OF THE INVENTION

In response to the aforementioned problems with known systems and theneed for a contactless interrogation technique, the present inventioncontemplates apparatus for contactless interrogation of an sensorintegrally disposed with a structure including coil means for couplingsignals across a gap between a sensing circuit having a sense coil andan interrogation circuit having an exciter coil connectable to avariable frequency supply, the sensing circuit comprising means forchanging current through the sense coil in relation to the sensoroutput; the interrogation circuit comprising means for detecting currentin the exciter coil induced by the sense coil current; and control meansfor determining the sensor output based on the exciter coil currentcompensated for the gap.

The invention also contemplates the methods employed in the use of suchapparatus, as well as a method for contactless interrogation of a sensorof the type that exhibits an output characteristic that changes in knownrelation to a physical parameter including the steps of:

a. using the sensor characteristic to determine a variable component ina reactive circuit;

b. applying a first frequency input signal to the circuit using magneticcoupling across a gap with the first frequency being in a range suchthat a first circuit output is independent of the variable component;

c. applying a second frequency input signal to the circuit with thesecond frequency being in a range such that a second circuit output isdependent on the variable component; and

d. determining the sensor output as a function of the first and secondcircuit outputs.

These and other aspects and advantages of the present invention will bereadily understood and appreciated by those skilled in the art from thefollowing detailed description of the preferred embodiments with thebest mode contemplated for practicing the invention in view of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a sensor interrogation apparatusaccording to the present invention for contactless interrogation ofsensors used in or on structures;

FIG. 2 is a detailed electrical schematic diagram of a circuit such asused with the apparatus of FIG. 1 that can be used to interrogate asensor embedded in or disposed on a structure;

FIG. 3 is a representative graph of primary current (as detected byvoltage across a sense resistor) vs. excitation frequency for thecircuit of FIG. 2; and

FIG. 4 is a block diagram of a neural net processing circuit that can beused with the sensor interrogation circuit of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, we show in diagrammatic form an apparatus 10that embodies the present invention for contactless interrogation of asensor or plurality of sensors embedded in a structure. Althoughspecific reference is made herein to embedded sensors, those skilled inthe art will readily appreciate that the term "embedded" is intended tobe interpreted in its broadest sense to include, for example, sensorsdisposed on a surface of or integrated with a structure. In addition,the term "contactless" is intended to be construed in its broadest sensewherein data and/or energy are coupled into and/or out of the structureby magnetic coupling rather than, for example, with electrical oroptical contacts. The particular structure involved in the use of theinvention may be any structure made of material compatible withapparatus and methods embodying the invention. Such materials includegraphite/epoxy composites, fiberglass, plexiglass and so on to name justa few. Clearly, this list is intended to be representative and notexhaustive of the possible choices of material available to theapplications designer. The material selected, however, should berelatively non-absorptive of the electromagnetic energy spectrum used tointerrogate the sensor(s).

The apparatus 10 includes a preferably surface mounted primary orinterrogation coil 12, and a secondary or sense coil 14. In the exampleof FIG. 1, the sense coil 14 is embedded in a structure, S, althoughtypically the sense coil will be disposed at or near a surface of thestructure. The sense coil is part of a resonant sensing circuit that isembedded in the structure and which, in combination with the othercomponents of the apparatus 10, is used for detecting the condition oroutput of a sensor dependent element 20 that is embedded in thestructure. In one example described herein, the sensor dependent element20 is a variable resistance strain gauge for detecting structural stressand strain forces that the structure S is subjected to. However, thisspecific example is intended to be exemplary and not limiting. Theinvention can be used with any sensor that produces a variableresistance output, or that produces an output that can be converted to avariable resistance, or that produces an output that can change ormodulate the value of one or more of the resonant circuit components. Asa further example, an optical sensor that produces an output consistingof modulated light beams can be interrogated by using the output lightto affect an impedance in the apparatus 10, such as a photosensitiveresistor.

As stated, the sense coil 14 is connected in a resonant sensing circuit22 that includes the sensor dependent element 20 and a capacitance 24.It is important to bear in mind that the variable resistance R used inFIG. 1 to represent the sensor 20, may in actual practice be aresistance that is affected by the output of the sensor, and not part ofthe sensor itself. The sensor itself can thus be disposed within thestructure S at any desired location an-connected to the resonant circuitby leads 26,28. For example, in the case of an optical sensor, thesensor output could be coupled to the sensor dependent element 20 byoptic fibers embedded in the structure with the sensor.

The sense coil 14 is preferably embedded in the structure at a knownlocation so that the interrogation coil 12 can be positioned in closeproximity thereto to achieve good magnetic coupling between the coils.For example, the coil 12 could be embedded in a conformal structure thatcan be placed over the structure S. A suitable arrangement for suchalignment is disclosed in the above-referenced co-pending U.S. patentapplication Ser. No. 981,966. As illustrated in FIG. 1, typically thesense coil 14 and interrogation coil 12 will be separated by a gap 30.This gap often will include the structural material involved in thesensor 20 analysis, or could simply be air or other non-magnetic medium,for example, in the case of surface mounted coils and sensors.

The gap 30 defines a distance "x" that separates the interrogation coil12 from the sense coil 14. This distance affects the quality of themagnetic coupling between the coils and consequently affects the abilityto detect the condition of the sensor and to couple the sensor data fromthe structure to an analyzer. According to an important aspect of theinvention, the gap x can be determined for each interrogation of thesensor, even on a real time basis, in order for the output signals to beadjusted or compensated for the particular gap distance existing at thetime of the interrogation. This is a substantial advance over priorknown systems because the ability to compensate for the gap permits theuse of amplitude variant signals for accessing the sensor output fromthe structure, rather than frequency variant signals. Thus, theinvention has significant benefits for use with optical sensors becausemany optical sensors produce outputs wherein the sensor data is encodedin light signals that are amplitude or intensity variant.

The interrogation or exciter coil 12 is part of an interrogation circuit40 that includes: a variable frequency energy source 42 which may, forexample, be a voltage controlled oscillator (sinusoidal or othersuitable application specific wave form); a load or sensing resistor 44(sometimes identified herein as R₄₄); a signal conditioning circuit 46;and a data processing and control circuit 48 that produces an output 50that corresponds to the sensor output or condition, and produces acontrol signal on line 52 for controlling the frequency and themagnitude of the excitation signal that the oscillator 42 applies to theinterrogation coil 12. The exciter coil 12, sensing resistor 44 andsource 42 provide a resonant exciter circuit 36 that is used to inducecurrents in the sense coil 14 by magnetic coupling between the coils12,14.

The basic operation of the exemplary apparatus 10 of FIG. 1 according tothe invention is as follows. The sensing circuit 22 exhibits a resonantfrequency (f_(s) in FIG. 3) which we define as the frequency which isthe point of maximum sensitivity to changes in I_(P) for a given changein R_(STRAIN) (maximum for ΔI_(P) /ΔR_(STRAIN).) The resonant frequencyf_(s) is determined by the sum total of the reactive elements of thecircuit which includes the inductance of the sense coil 14 and theinterrogation coil 12, as well as the capacitance 24 (and parasiticcapacitances C_(P1) and C_(P2) shown in FIG. 2) and the value of K. Theamplitude of the current through the coil 14 is also a function of thesensor dependent element 20, particularly at the resonant frequency ofthe sensing circuit 22. When the exciter coil 12 has an AC signalapplied, current in the exciter or primary coil 12 induces current inthe secondary or sense coil 14, as in an air gap transformer. Thiscurrent in the sense coil, however, is reflected back to the excitercoil 12 by the mutual coupling of the two coils. The sense resistor 44is used to detect the current in the exciter coil 12. When theexcitation frequency is approximately at the resonant frequency of thesensing circuit 22, the current in the exciter coil changes maximally inrelation to the value of the sensor dependent element 20. Thus, thesensor condition can be determined as a function of the detected currentin the exciter coil. The signal conditioning circuit 46 is used toamplify the voltage developed across the sense resistor 44 by theexciter circuit current. This amplified voltage is then rectified andlow pass filtered to provide a DC voltage output. The control circuit 48then uses the DC value to determine the state or output of the sensor20.

As noted herein, the amplitude of the current that flows in the coils isdependent on the value of the sensor dependent element 20. The coilcurrents, however, are also strongly a function of the size andcharacteristics of the gap 30. This is because the gap, and the mediumin the gap, between the coils proportionately affects the magneticcoupling between the two coils. For example, the coupling constant, K,between the coils is directly proportional to the inverse distance x.Therefore, the magnitude of currents induced in the coils is a functionof the distance x. This has been one of the major reasons why amplitudebased measurements have heretofore been impractical, because precisecontrol of the gap 30 is difficult, and in fact near impossible when itis desired to have the external interrogation circuit movable betweendifferent sensor locations in a structure. In accordance then with animportant aspect of the invention, the control circuit 48 is configuredin a manner such as set forth hereinafter to characterize the gap 30 andthus accurately determine the sensor condition based on amplitudevariant signals from the exciter circuit.

With reference next to FIG. 2, we show a more detailed circuit model ofan exciter circuit 36 and sensing circuit 22 useful in practicing theinvention. As shown, the exciter circuit 36 includes the exciter coil 12that has a determinable inductance, L_(P). The coil 12 and associatedcomponents of the exciter circuit 36 also will exhibit an overallparasitic capacitance, C_(P1), that appears in parallel with the coilinductance. The exciter circuit further includes the variable frequencyoscillator 42 and the sensing resistor 44 used to sense the primary orexcitation current I_(P). Thus, all components in the exciter circuit 36are known quantities for each application.

The resonant sensing circuit 22 includes the sense coil 14 which has adeterminable inductance, L_(s). The sense coil 14 also has an associatedparasitic capacitance, which parasitic capacitance is in effect part ofthe capacitance C_(P2) which is a discrete capacitor selected tooptimize the sensitivity of the apparatus 10 to changes in the value ofthe element 20. In other words, the value of C_(P2) can be selected,such as based on experimental data for specific circuits, to maximizethe current I_(P) induced in the exciter circuit 36 as a function ofchanges in the resistance R_(STRAIN) (e.g. maximize the ratio ΔI_(P)/ΔR_(STRAIN).) The sense circuit 22 also includes an additional discretecapacitor C which is selected to adjust the frequency at which theΔI_(P) /ΔR_(STRAIN) ratio is optimized. Thus, for the sense circuit 22,all of the component parameters are known quantities except the couplingconstant, K, and the value of the sensor output (as represented by theunknown quantity R_(STRAIN) in the specific example of the describedembodiment, but more generally the value of the sensor dependent element20).

FIG. 3 is a graph showing in a representative manner a typical frequencyresponse characteristic of the circuit of FIG. 2, as shown by the familyof curves determined by monitoring the primary current I_(P) vs.excitation frequency for different K values (in this example for K=0.1,K=0.5 and K=0.9) and different resistance values for the sensordependent element 20. Note that the Y-axis is a logarithmic scale. Inthis example, as in the system embodiment of FIG. 1, the current I_(P)is detected as a voltage developed across the sense resistor 44, withthis voltage being rectified to a DC value.

Several important attributes of the circuit should be noted. Graph Acorresponds to the frequency response for K=0.1, which may, for example,correspond to a rather large gap (high value of "x" on the order ofapproximately 1/2 the coil diameter) with reduced magnetic couplingbetween the coils 12,14. Virtually no sensitivity is present todifferent values of the element 20. In other words, regardless of theexcitation frequency, the current in the exciter circuit 36 is not adetectable function of the value of the element 20. It will further benoted that as excitation frequency increases, the current I_(P)initially decreases, indicative of the exciter circuit 36 resonantimpedance; however, if the frequency sweep were extended beyond 20 MHz(not shown in FIG. 3), the current I_(P) would reach a minimum at theresonant frequency of the L_(P) C_(P1) circuit and then increase to amaximum value approximately equal to V₄₂ /R₄₄ (the source voltagedivided by the sense resistor.)

Graph B corresponds to the frequency response for K=0.5, which may, forexample, correspond to a moderate gap (intermediate value of "x" on theorder of approximately 1/10 the coil diameter) with somewhat reducedmagnetic coupling between the coils 12,14. First note that at afrequency generally designated f_(c), the circuit exhibits somesensitivity to the value of the resistance of the element 20. Thisoccurs, for example, around 9-10 MHz. At higher frequencies, forexample, at the frequency generally designated f_(s), the circuitexhibits substantially increased sensitivity to the value of the element20, for example, around 11 MHz. Further note that at lower frequencies,for example around 5 MHz, the frequency response is again independent ofthe value of the resistance of element 20. The frequency f_(c) can beapproximated by the formula 1/(2π*SQRT(L_(S) [C_(P2) +C])).

Graph C corresponds to the frequency response for K=0.9, which may, forexample, correspond to a small gap (short value of "x" on the order ofless than 1/10 the coil diameter) with a high degree of magneticcoupling between the coils 12,14. First note that at approximately thesame frequency generally designated f_(c), the circuit exhibits somesensitivity to the value of the resistance of the element 20, and agreater sensitivity than when K=0.5. At higher frequencies the circuitagain exhibits substantially increased sensitivity to the value of theelement 20, for example, at a frequency generally around 20 MHz. Furthernote that at lower frequencies, for example around 5 MHz, the frequencyresponse is again independent of the value of the resistance of element20.

Also it should be noted that at the lower frequency range, such as at 5MHz, the voltage detected across the sense resistor 20 is dependent onthe value of K, but independent of the value of the resistance 20.Therefore, the value of K can be determined by applying a firstexcitation frequency, such as 5 MHz, to the exciter coil 12, anddetecting the resultant current I_(P). Having determined the value of K,the value of the element 20 can be determined by applying a secondexcitation signal at a frequency and magnitude range wherein the circuitis known to exhibit maximum sensitivity to the value of the element 20.The second frequency can be selected in at least two ways. First, asshown in FIG. 3, the frequency f_(c) is not strongly dependent on thevalue of K. Therefore, the first and second excitation frequencies cansimply be fixed values based on the particular circuit components usedin an application. Alternatively, sensitivity and accuracy can beincreased by adaptively selecting the second frequency based on thedetermined value of K (from the measurement made at the first excitationfrequency.) That is, the second frequency can be selected so that itcorresponds to a circuit response at which the current I_(P) is morestrongly dependent on the value of the element 20. As is clear from FIG.3, however, the value of this second frequency depends on the value ofK. Therefore, the control circuit 48 is configured to determineadaptively the correct value of the second excitation frequency for eachpossible value of K.

A mathematical model of the circuit of FIG. 2 can be derived fromfundamental circuit theory: ##EQU1##

The only quantities in equation 1 that are unknown after the circuitcomponents have been selected are K and R_(STRAIN). Thus, if twomeasurements of I_(P) are made at two different frequencies with onefrequency being selected so that I_(P) is independent of the value ofR_(STRAIN), and the other frequency selected so that I_(P) is dependenton the value of R_(STRAIN), then both unknown quantities can bedetermined.

With reference again to FIG. 1, it is now apparent that the controlcircuit 48 is configured to determine first the value of K by applying afirst frequency to the exciter coil 12, and detecting the resultantvoltage produced across the sense resistor 44. This voltage thenindicates the value of K, from which the control circuit 48, for theadaptive processing method, selects a second frequency to apply to theexciter circuit such that the resultant voltage measured across thesense resistor corresponds to the value of the element 20.

A particular advantage of the invention is that an embedded sensor canbe energized, interrogated and the sensor information coupled in acontactless manner to a processing circuit through the use of a singlecoil pair.

The control circuit 48 can be realized in the form of a microprocessoror similar controller that accesses data stored in memory based oncircuit characterization data and look up tables so as to determine thevalues of K and R_(STRAIN) and produce a corresponding output 50. Thiscan be accomplished in a conventional data processing manner forextrapolating data based on empirically derived characterization values.For example, the specific circuit used for an application can becharacterized based on actual frequency response data for differentvalues of K and the element 20. This data can then be stored in look uptables for processing by the control unit 48. Although this approach maybe feasible for some applications, other more complicated applicationsmay require an extensive amount of test data to sufficientlycharacterize the circuit so that accurate values of K and the element 20can be determined. This extensive data also requires large amounts ofmemory for access by the control circuit 48.

In accordance with another important aspect of the invention then, wehave found that neural net processing can be used to implement either orboth of the processing methods described above, i.e. the determinationof K and R_(STRAIN) based on optimized selections of the first andsecond excitation frequencies, or the adaptive processing technique inwhich the second excitation frequency is selected as a function of thevalue of K. We have further found that while the values of K andR_(STRAIN) can be determined with good accuracy, we can substantiallyimprove accuracy by using neural processing to calculate directly thevalues of "x" and strain, ε. Of course, this is only an exemplarydescription. Strain is just one of many parameters that can bedetermined, and as previously described the element 20 can be used as atransducer to convert optical sensor output signals into resistancevariations that can be determined by the neural processing. Forconvenience we will continue with the example of using the concepts ofthe invention for strain measurement, but such example is not to beconstrued in a limiting sense.

With reference to FIG. 4, a suitable architecture is illustrated for aneural net processing methodology to determine the values of "x" andstrain, ε, or alternatively the values of K and R_(STRAIN). Basically,the neural net includes three subnets identified 60, 62 and 64. Thefirst subnet 60 is used to determine the value of "x" based on thevoltage reading across the sense resistor 44 at a first excitationfrequency, ω₁. The second subnet 62 is used to determine the optimalvalue of the second frequency, ω₂, about which the circuit exhibitsmaximum sensitivity to the changes in the element 20. An adaptivemeasurement at 63 is used to select an appropriate excitation voltage atfrequency ω₂ (for the source 42.) The magnitude is selected so that thesignal processing is operated full range to provide maximum outputsignal at the minimum value of R_(STRAIN). The selection of theexcitation signal magnitude can be conveniently incorporated into theneural network training process. The third subnet 64 is used todetermine the value of the sensor output (in this case strain) as afunction of the voltage measured when the second excitation frequency isapplied.

The neural nets are developed and trained in a conventional manner. Datais collected for the circuit frequency response across a selectedfrequency range while changing the values of "x" and strain in a knownmanner. This data is then used to train the neural nets. Accuracies ofbetter than 1% have been achieved.

The neural nets used were generated following their detailed descriptionin "Structure-unknown non-linear dynamic systems: identification throughneural networks" by S. F. Masri, A. G. Chassiakos and T. K. Caughey inJournal of Smart Materials and Structures, Vol. 1., No. 1 pgs. 45-56.The nets utilized a single input which was preprocessed into componentsof an orthogonal polynomial basis set, i.e., X→X, X², X³, . . . , X⁸ sothat 8 neurons comprised the fan out layer. The nets had two hiddenlayers having 10 neurons each and an output layer having one or twoneurons depending upon requirements. The neurons utilized signoidactivation functions. A listing of a training algorithm for one of thenets having a single output neuron is appended hereto. The algorithm iswritten in the Microsoft Quickbasic® computer language.

While the invention has been shown and described with respect tospecific embodiments thereof, this is for the purpose of illustrationrather than limitation, and other variations and modifications of thespecific embodiments herein shown and described will be apparent tothose skilled in the art within the intended spirit and scope of theinvention as set forth in the appended claims.

We claim:
 1. Apparatus for contactless interrogation of a sensorintegrally disposed with a structure comprising: coil means for couplingsignals across a gap between a sensing circuit having a sense coil andan interrogation circuit having an exciter coil connectable to a supply,said sensing circuit operating as a load that changes in relation to thesensor output; said interrogation circuit comprising means for detectingamplitude of a signal responsive to said load and induced in saidexciter coil across said gap; and control means for determining thesensor output based on said detected amplitude compensated for said gap.2. The apparatus of claim 1 wherein said sense coil current is amplitudemodulated in relation to the sensor output.
 3. The apparatus of claim 1wherein said control means applies a first operating frequency to saidexciter coil for determining a parameter related to said gap, andapplies a second operating frequency to said exciter coil fordetermining the sensor output, said second frequency being selected as afunction of said gap parameter.
 4. The apparatus of claim 1 wherein saidsense coil and said exciter coil are magnetically coupled fortransmitting sensor related signals between said coils.
 5. The apparatusof claim 4 wherein said sensing circuit is a resonant circuit comprisingsaid sense coil and a capacitance, said sensing circuit furthercomprising a sensor dependent element that varies current through saidsense coil in relation to the sensor output.
 6. The apparatus of claim 4wherein said interrogation circuit comprises said exciter coil and anexciter coil current sensing means, said current sensing means producingan output related to current through said sense coil.
 7. The apparatusof claim 6 in combination with a strain gauge connected to said sensingcircuit and having a resistance characteristic that modulates currentthrough said sense coil.
 8. The apparatus of claim 7 wherein the sensorand said sensing circuit are embedded in a composite structure.
 9. Theapparatus of claim 8 wherein said interrogation circuit is external tothe structure.
 10. A method for contactless interrogation of a sensor ofthe type that exhibits an output characteristic that changes in relationto a physical parameter comprising the steps of:a. using the sensorcharacteristic to determine a variable component in a reactive circuit;b. applying a first frequency input signal to the circuit using magneticcoupling across a gap with said first frequency being in a range suchthat a first circuit output is independent of the variable component; c.applying a second frequency input signal to the circuit with said secondfrequency being in a range such that a second circuit output isdependent on the variable component; and d. determining the sensoroutput as a function of said first and second circuit outputs.
 11. Themethod of claim 10 wherein said step of applying a second frequencyinput signal includes the step of selecting said second frequency andthe magnitude of said second frequency input signal as a function ofsaid first circuit output.
 12. The method of claim 11 wherein the stepof applying a first frequency input signal includes the step ofselecting the first frequency in a range such that said first circuitoutput signal is dependent on a parameter of said gap.
 13. The method ofclaim 12 wherein the step of using magnetic coupling across a gapincludes the step of using two coils magnetically coupled to each otherto interrogate the sensor by varying and detecting current through thecoils in relation to the sensor output.
 14. The method of claim 13wherein the step of using the sensor characteristic to determine avariable component includes the step of using the sensor to vary aresistance in the circuit such that current through said coils dependson the resistance at said second frequency.
 15. The method of claim 14including the step of using a resistive strain sensor as the variablecomponent.
 16. The method of claim 10 wherein said steps of selectingsaid first and second frequencies, and determining the sensor output areeach performed by a respective trained neural network.
 17. The method ofclaim 10 wherein said circuit input and output signals are coupled toand from the circuit using two substantially identical coils separatedby said gap.
 18. The method of claim 17 further comprising the step ofusing RF frequency input signals coupled to the circuit from an excitercoil to a sense coil.
 19. The method of claim 18 wherein said outputsignals comprise signals induced in said exciter coil by current flowingin said sense coil.
 20. The method of claim 19 wherein the physicalparameter is detected using a resistive element having a resistance thatis a function of the parameter.
 21. The method of claim 20 wherein theparameter is strain.
 22. The method of claim 19 wherein the variablecomponent is a resistive strain gauge.
 23. The method of claim 22wherein the variable component is a resistive semiconductor straingauge.
 24. Apparatus for contactless interrogation of a sensor of thetype that exhibits a characteristic that changes in known relation to aphysical parameter, the apparatus comprising reactive circuit meansincluding a component having a value that is a function of the sensorcharacteristic; coupler means for applying first and second inputsignals to said circuit means at two different frequencies across a gapand for detecting first and second output signals produced by saidcircuit means; and control means for controlling said input signals andinterpreting said output signals; said first output signal being relatedto said gap independent of said component value, and said second outputsignal being related to said component value.
 25. The apparatus of claim24 wherein said coupler means comprises an exciter coil and a sense coilseparated by said gap, said exciter coil being connectable to a variablefrequency energy source, said input signals being inductively coupledfrom said exciter coil to said sense coil.
 26. The apparatus of claim 25wherein said output signals comprise signals induced in said excitercoil by current flowing in said sense coil.
 27. The apparatus of claim24 wherein said control means receives said first output signal,determines a parameter related to said gap, and selects said secondfrequency as a function of said gap parameter.
 28. The apparatus ofclaim 27 wherein said control means selects said first frequency in arange in which said circuit means produces an output independent of saidcomponent value.
 29. The apparatus of claim 28 wherein said controlmeans selects said second frequency in a range in which said circuitmeans produces an output related to said component value.
 30. Theapparatus of claim 29 wherein said component is a variable resistanceand said circuit means comprises a RLC circuit that exhibits a resonantfrequency at which current in said sense coil is a detectable functionof said resistance.
 31. The apparatus of claim 24 wherein said controlmeans comprises neural net means for determining a gap parameter as afunction of said first output signal, for selecting said secondfrequency as a function of said gap parameter, and for determining saidcomponent value as a function of said second output signal.
 32. Theapparatus of claim 31 wherein said neural net means comprises threeneural subnets for respectively determining said gap parameter,selecting said second frequency and determining said component value.33. The apparatus of claim 1 wherein said supply provides an excitationsignal of selectable frequency to said exciter coil, and further whereinsaid induced load responsive signal has a frequency that substantiallymatches said excitation signal frequency.